The present invention relates to a rear-wheel steering apparatus for a vehicle for turning rear wheels upon a turning operation of front wheels, for example, a rear-wheel steering apparatus for turning rear wheels in a phase opposite to that of front wheels when the front wheels are turned at a large turning angle.
As a rear-wheel steering apparatus for a vehicle, two mechanisms, i.e., turning angle sensitive type and vehicle speed sensitive type mechanisms are employed. In the former mechanism, when front wheels are turned at a small turning angle, the rear wheels are turned to be in-phase with the front wheels. When the front wheels are turned at a large turning angle, the rear wheels are turned in a phase opposite to that of the front wheels. In the latter mechanism, the front and rear wheels are turned in opposite phases at a low vehicle speed, and are turned to be in-phase with each other at a high vehicle speed.
As a conventional turning angle sensitive four-wheel steering apparatus, a technique disclosed in U.S. Pat. No. 4,295,657 is known. The four-wheel steering apparatus disclosed in this prior art comprises a cam plate which is displaced along the back-and-forth direction. A substantially S-shaped cam groove is formed in this cam plate. A pin fixed to a rod for turning the rear wheels is slidably engaged with the cam groove. In this manner, the rear wheels can be turned to the right or left.
However, in the four-wheel steering apparatus disclosed in U.S. Pat. No. 4,295,657, a large force acts on a pin engaged with the cam groove, and as a result, a sliding resistance is increased, thus making smooth control difficult. In addition, the mechanical strength of the pin is decreased, resulting in poor reliability and durability. Upon engagement between the pin and the cam groove, a gap (clearance) is inevitably formed between the pin and the cam groove. As a result, backlash occurs between the pin and the cam groove, and this leads to degraded control precision.
As still another prior art of the turning angle sensitive four-wheel steering apparatus, one disclosed in Japanese Utility Model Laid-Open (Kokai) No. 62-71080 is known. In the four-wheel steering apparatus disclosed in this utility model, an arcuated cam groove is formed in a side surface of a rotating cam plate. A pin fixed to a rod for turning the rear wheels is slidably engaged with the cam groove. In this manner, the rear wheels can be turned to the left or right.
However, the four-wheel steering apparatus disclosed in Japanese Utility Model Laid-Open (Kokai) No. 62-71080 mainly comprises a cam groove and a pin engaged therewith, and has problems similar to those of the conventional four-wheel steering apparatus disclosed in U.S. Pat. No. 4,295,657.
As a turning angle sensitive four-wheel steering apparatus which does not employ an arrangement comprising a cam groove and a pin engaged therewith, techniques disclosed in Japanese Patent Laid-Open (Kokai) Nos. 63-97468 and 63-97469 are known. In these prior arts, a spherical bushing (15) is urged against an outer peripheral surface of a rotating cam plate, and is provided to a follower lever (10), one end of which is pivotally supported. A rear-wheel tie rod (13) is coupled to the follower lever, and upon left or right swinging movement of the follower lever, the rear wheels are turned to the left or right.
However, in the conventional arrangement, a space for swinging the follower lever must be kept, and the apparatus may become bulky. On the follower lever, the position of the spherical bushing and the coupling position of the tie rod are separated in the back-and-forth direction. As a result, a bending moment acts on the lever, and the mechanical strength of the lever must be increased to cope with it. As a result, the lever is increased in size, and its weight is also increased.